Sustained low dissolved oxygen levels in an aeration tank can cause a host of problems — including difficulties in maintaining soluble BOD (biochemical oxygen demand), reduced nitrogen removal efficiency, and challenges with biomass separation.
A jet aeration system offers a solution, utilizing both a pump and blower to meet the oxygen transfer requirements of a biological process. A well designed jet aeration system should provide more than 20 years of reliable service. However, proper maintenance is required. Issues such as reduced pump flow and clogged jet nozzles can interfere with effective oxygen delivery.
Here are five steps that should be carried out if you have low dissolved oxygen levels.
Make sure the jet pumps and blowers are running at full speed: Measure blower and pump amps and compare to start-up conditions. It is also important to check blower discharge pressure, discharge temperature, and physically check the inlet filters for cleanliness.
Closely observe the surface aeration pattern and look for changes: Check for large surface boils directly above the aeration manifold. This may indicate clogged jet nozzles. A more uniform uncharacteristic, larger bubble surface pattern may indicate that the jet pump is not delivering the design liquid flow, or it could indicate the presence of antifoam or defoamer, which can cause bubble coalescence.
Compare suction and discharge pump pressure gauges to start-up conditions: If the discharge pressure is reading high this could indicate a blockage in the discharge piping, aerator liquid pipe, or jet nozzles. If the suction gauge reading is low this could indicate a restriction in the pump suction piping.
Open the air relief valve: This is located at the top of the pump discharge and suction piping outside the tank. If you can hear a rush of air coming out, this could indicate the pump was air bound, which will restrict pumping capacity.
Check to see if there have been changes to the Clean-in-Place (CIP) chemicals: While this step only applies to industrial plants, the addition of an antifoam or defoamer in the production facility or treatment plant or polymer in biomass separation processes, can contribute to a chronic low dissolved oxygen (DO) problem. Many antifoams and defoamers can reduce the oxygen transfer capacity of an aeration system by more than 50 percent. Excessive polymer additions can increase the viscosity of the mixed liquor suspended solids (MLSS), which in turn adversely affects oxygen transfer.
